The present invention relates to building construction and more particularly to improved structures and buildings of prestressed concrete, reinforced concrete and the like.
Many buildings, such as offices, meeting and concert halls, churches, theatres, and parking structures require large floor spans because of functional requirements. A large number of such structures of the prior art have heavy beam and slab floor construction, comprising relatively deep beams of about three foot depth supporting a deck slab, the beams in turn being supported by columns.
The beam and slab construction of the prior art is either cast in place, or "precast" post-tensioned concrete consisting of large, heavy elements delivered to and assembled on the site, or steel beam and column elements supporting a concrete deck.
This type of construction has significant problems. For example, the deep beams hang down over the floor below, restricting the space between floors. This problem is accentuated in parking structures, where the columns need to be widely separated. In parking structures the floors normally span over 60 feet to allow a stall twenty feet in length on each side of a driveway, plus 23 feet minimum for the driveway. In a typical parking structure having beam and slab construction, the floor beams or girders, spaced 10 to 25 feet apart must be 30 to 40 inches deep. Since the floors are only 10 feet apart vertically, the volume required for the floor construction is about 1/3 of the height of the parking structure. This is lost space that cannot be utilized, and money has to be paid for this wasted space by the owner of the parking structure.
Another disadvantage of current beam and slab construction is restricted headroom. About one out of four private vehicles in many localities are vans and small pick-up trucks that are excluded from most existing parking structures. New laws for handicapped easy access are being passed that cannot be enforced without making the parking structures much more expensive. The floor-to-ceiling height must be raised by more than 18 inches to permit access of vehicles transporting the handicapped (usually vans, small buses, pick-ups).
Another disadvantage is that these beam-slab structures are very expensive, laborious and time-consuming to construct, especially the cast-in-place concrete or structural steel versions.
In the case of cast-in-place concrete which is normally post-tensioned, a large number of reinforcing devices in the form of stirrups, bars, tendons, and the like have to be formed and placed individually with progressive spacing and tied together to form a cage of reinforcement before the concrete is poured in the formwork. Thus forming of the beams is expensive, time-consuming, laborious and complicated. The placement and tying together of the individually bent and progressively spaced reinforcement members are particularly difficult to supervise and inspect, greatly increasing the cost of the building.
The precast post-tensioned type of construction introduces other disadvantages. Especially serious is that it relies on "dry" connections between beams and columns, wherein the beams rest on small bearing areas or corbels that protrude from the columns. A relatively modes tremor can introduce sufficient lateral displacement to the columns to remove a small bearing area from the beams, allowing the floor to come crashing down.
Another method for constructing buildings, called "flat slab construction", is to leave out the beams. The beams can be left out by reducing the spacing between columns. The flat slab construction lends itself to modern production techniques, such as fly-forming, wherein a portable, flat-top form is positioned to define the bottom surface of poured concrete for the floor. Once the concrete has set, the form can be relocated to another floor region and re-used. The floor, which can include steel reinforcing bars and/or post-tensioning tendons, is typically eight or ten inches thick.
A disadvantage of flat slab construction is that the column spacing must be about 30 feet or less, even in lightly loaded structures. In a parking structure, for example, the effective live load is only about 30 pounds per square foot, but the floor itself contributes more than 100 pounds per square foot to the total load. Increasing the floor thickness is impractical because of the extra weight and expense of the added material. Thus, flat slab construction is unsuited to many building applications demanding spans of 60 feet or more.
A further disadvantage of each of the prior art building structures is that a great proportion of the weight of the floor is located near the center of the slab, farthest from the columns. Consequently, the building has a low resistance to vibration, especially in a vertical direction.
Accordingly there is a need for a building structure and method that permits large floor spans without requiring deep floor beams, that is safe and reliable, and inexpensive to produce.